The present invention relates to a solid polymer electrolyte membrane usable for a solid polymer electrolyte fuel cell (PEFC), particularly to a solid polymer electrolyte membrane that is excellent in sealing properties to be capable of preventing leakage of a fuel gas (hydrogen gas), an oxidant gas (air), a humidification water and a coolant in a fuel cell. The present invention also relates to a fuel cell comprising the solid polymer electrolyte membrane.
In general, a fuel cell is provided by stacking a plurality of fuel cell units, a separator being disposed between the fuel cell units. Each of the fuel cell units comprises a solid polymer electrolyte membrane, an anode disposed on one surface of the membrane, and a cathode disposed on another surface of the membrane. More specifically, such a stacked-type fuel cell comprises: a carbon separator with electron-transporting properties having passages for independently introducing a fuel gas, an oxidant gas and a coolant to each fuel cell unit; a carbon fiber diffusion layer that diffuses the fuel gas or the oxidant gas and comes into contact with a convex part of the carbon separator to transfer electrons between an electrode and the carbon separator; an anode where the fuel gas is subjected to a chemical reaction to provide protons and electrons; a cathode where water is generated from oxygen, protons and electrons; and an electrolyte membrane in a wet state for transporting protons.
The fuel gas and the oxidant gas are used for the fuel cell as reaction gases, the fuel gas is supplied through an anode side passage of the separator, and the oxidant gas Is supplied through a cathode side passage of the separator. When each fuel cell unit is supplied with the reaction gases, the electrochemical reaction proceeds to generate electrons and the electrons are utilized in an external circuit as an electric energy.
The fuel gas, the oxidant gas and the coolant should be independently supplied to the fuel cell unit through different passages, therefore, it is important to seal the passages. Sealing method can be selected from various methods depending on the structure of the stacked fuel cell units. For example, a sealant may be disposed: around a communicating aperture going through the fuel cell stack for supplying the fuel gas, the oxidant gas, the humidification water and the coolant to each fuel cell unit; on the periphery of MEA (the electrolyte membrane+the electrodes+the diffusion layer); on the periphery of the passage where the coolant is supplied along surfaces of the separator to cool the separator; on the periphery of the separator; etc.
Known as the sealing method are: (i) methods where the fuel cell units and a frame having a sheet-shape, an O-shape, etc. are stacked while pressing, the frame being made of an elastic material such as an organic rubber (a fluoro-rubber, a silicone rubber, ethylene-propylene rubber, etc.) and an adhesive If hardening type-liquid material, thereby utilizing repulsive force of the elastic material to seal the fuel cell; (ii) methods where the fuel cell is compressed and sealed by an inorganic sheet such as a fiber sheet of graphite, ceramic, etc.; (iii) methods using a caulking or a mechanical sealant; etc.
Though a material for the sealant and a shape of the sealant should be selected in accordance with strength, surface state, etc. of the separator or MEA, the sealant is preferably miniaturized in the case of equipping an automobile with the fuel cell. In particular, each fuel cell unit of the fuel cell has to be thinned, thus, MEA and the separator have to be thinned. The separator, with which the sealant directly comes into contact, is generally made of a brittle material such as carbon, etc., so that the thinned separator is often broken when it is stacked with the fuel cell units. Thus, among the above methods of (i), (ii) and (iii), preferred are the methods of (i) using the sealant having proper elasticity and repellency.
However, when the fuel cell units and the separator are sufficiently sealed while pressing such that the separator is not broken, surface states such as a crease, a swell, a fold, a bend, a roughness, etc. of the MEA, particularly the electrolyte membrane coming into contact with the sealant, is remarkably affecting the sealing properties.
The electrolyte membrane in MEA is disadvantageous in that it abruptly expands or shrinks correspondingly to moisture content of air. The portion of MEA that comes in contact with the sealant is composed of only the electrolyte membrane without the electrodes and the diffusion layer, and the electrolyte membrane is often creased by the sealant. Thus, it is difficult to secure sufficient sealing properties even if the material and structure of the sealant are properly selected. Further, strict humidity control is required to assemble the MEA into the fuel cell without creasing the electrolyte membrane, thus, the conventional sealed fuel cells are poor in productivity.